In the semiconductor industry, wafer bonding is conventionally performed by methods where a first substrate is bonded to a second substrate and then the bulk of the first substrate is removed leaving a thin layer from the first substrate on the surface of the second substrate. The first substrate is usually referred to as the donor wafer, while the second substrate is referred to as the handle wafer. The transfer of the thin film from the donor wafer to the handle wafer can be achieved by known methods such as wafer back grinding, or hydrogen blistering (a process known as SmartCut). In all of these wafer bonding techniques, a continuous film is transferred from a donor wafer to a handle wafer, and the bonding takes place between two wafers.
Wafer bonding carried out by the techniques discussed above presents many challenges. For example and in conventional techniques, the transfer of the thin film from the donor wafer to the handle wafer requires a high temperature anneal (on the order of about 350° C. or greater). Thus, if the SmartCut approach is used, a first anneal is required to strengthen the bonding between the two wafers, while a second anneal at a higher temperature is required to activate the hydrogen blistering. No room temperature process for layer transfer has been reported so far.
Particles can substantially impact the yield of wafers bonded by conventional methods. As an example, a single particle with a radius of 0.1 micrometer can form a void (i.e., an unbonded area) of 1.0 millimeter radius. Thus, an ultra clean environment is required to obtain a void-free bonded wafer.
In addition to the particle-free surface requirement, the surfaces of the donor and handle wafers are required to be very smooth to enable bonding. A typical surface roughness of less than 0.5 nm (RMS) is usually specified. Some deposited films exceed a surface roughness of 0.5 nm, and a chemical mechanical polishing (CMP) step is used to smooth the surface prior to bonding.
Achieving a clean interface between the transferred film and the handle wafer is important if the transferred film is to be electrically connected to the substrate. As an example and in the case of silicon, hydrophobic bonding is required to obtain an Ohmic contact between the transferred film and the substrate. The silicon donor wafer and the silicon handle wafer surfaces are stripped of any oxide and then passivated to prevent the growth of a native oxide or otherwise the bonding interface will include an oxide film. The passivation of the bare silicon surface, typically achieved by a hydrofluoric acid (HF) last dip, is volatile and prevents oxide growth for only a short period of time (e.g., about 20 min at room temperature). Thus, the time window for bonding the two silicon wafers is very short.
Given the above challenges with prior art wafer bonding processes, a bonding and film transfer method that can be performed at a low temperature (e.g., room-temperature), is less sensitive to particles and surface roughness, and provides a clean bonding interface is highly desirable.